Tuberculosis (TB) is one of the most important causes of infectious morbidity and mortality worldwide. Young children are more likely to contract infection with and develop severe disease from the causative agent Mycobacterium tuberculosis (Mtb). These clinical observations likely reflect fundamental differences in the immune systems of young children and adults. Critical differences relevant to TB immunity include the propensity for infants and young children to develop TH2-type CD4+ T cells in response to immunogens, deficiencies in the development of TH1-type T cells in response to pathogens, and deficiencies in macrophage and dendritic cell (DC) function. We propose to systematically define, in young children (less than or equal to 10 years of age), these important differences relevant to the successful containment of Mtb infection. The specific aims are: 1) To determine if severity of disease following Mtb infection in young children is associated with TH2-type Mtb-specific immunity, and conversely, if absence of disease following Mtb infection is associated with TH 1-type Mtb-specific immunity. 2) To determine if immunologic immaturity is associated with the development of TH2-type Mtb-specific immunity following Mtb infection, and conversely if immunologic maturity is associated with the development of TH 1-type Mtb-specific immunity following Mtb infection. 3) To evaluate aspects of innate immunity relevant to Mtb infection by characterizing the phenotype and function of macrophages and DC from cord blood derived from healthy neonates in comparison to macrophage and DC function in healthy adults. These studies may contribute to a more complete understanding of TB immunity in young children, and hence 'facilitate the development of an improved TB vaccine for this vulnerable population.